Loud-speaker with a tapered horn coupled to the speaker diaphragm



F. MASSA LOUDSPEAKER WITH A TAPERED HORN COUPLED July 22, 1952 TO THESPEAKER DIAPHRAGM Filed June 12, 1948 rflv FIG. 2

. 3nventor Fag-5.7

5N CYCLES FREQUENCY Fla-1.8

Patented July 22, 1952 UNITED STATE s PATENT" oFFlcs LOUD-SPEAKER WITH ATAPERED HORN COUPLED TO THE SPEAKER DIAPHRAGM Frank Massa, ClevelandHeights, Ohio Application June 12,1948, Serial No. 32,607

9 Claims;

My invention is concerned with loud speakers and, more particularly,with loud speakers of very-small size such as aregenerally employed inthe so-called personal types-of battery operated portable radioreceivers." Due to the space limitations in thesesmall sizeradio sets,the dimensions of the loud speaker must necessarily bevery small. Loudspeakers employing diaphragmshaving a vibrating surface corresponding tothe area of 'a 2" diameter disc, more orless, are typical-of theloudspeakers generally employed for this application: Because of thesmallsizeof the vibrating diaphragm, the acoustic efiiciency and thecorresponding power output from these loud speakers-is-very low andsince the small portable radios which employ this type of loud speakerare operated with batteries,

a considerable drain onthe battery power isnecessitated if areasonableacoustic power out put is to be established. Another "disadvantage ofthe tiny loud speaker is that the resonant frequency'of the-vibratingdiaphragm is generally in the neighborhoodof 300'cycles or higher and,-as'a result, thereis a complete absence of any low-frequencyreproduction from the loud speaker, resulting in the well-knownftinny-"* quality of reproduction that-istypical of this type'radio set.

The general object of my invention is to imgprove both the efficiency ofreproduction of smallsize loud speakers and also -ito increase theirlow-" frequency response;

Another object of thisinvention istoprovide' a low-cost loud speakerdesign which permits the economical use of my new loud speaker system ina small-size portable radio set-.

Still another object of this invention-is'to pro--- vide a design suchthat the space'requirement for my improved'loud speaker will be kept-ata minimum.

A further object of my inventionis to reduce the resonant frequency ofthe vibrating system of the loud speaker by means externalto thediaphragm structure, thereby resulting in increased low-frequency range ofreproduction without sacrifice in the high-frequency performance.

One method for increasing the low-frequency response of a loud speakerwould'beto reduce the natural resonance frequency of the vibratingdiaphragm by the addition'of a weight tothemovingstructure, but if thismeans were em"- ployed, there would bea lowering ofthe mid'" andhigh-frequency sensitivity of the loud speak er because'ofthe addedweight'that would have to be set into vibration by the drivingmechanism,

thus resultingdn 'still further "r educed- "eiiji'cien'c'y ofthe loud"speaker over frequency range." f.

In one specificembodl I provide an incrasf surface'of the diaphragm"quencies, so that the r "so the"diaphragm"is material lat same time theeffective inassbith diaphragmat' the higher frequencies}, unchanged; Iachieye v tli ing an air chamber-i111 diaphragm operatesfwhlfetrueexponenti'al hornat but which acts-as an acous the lowerfrequencies.

In one form of my invention midget-size loud speakrfifl coup shaped airchamber whi'c "(its actance at the low frequencie erv mfiifi ssubstantially w at the higher frequencies toine'reasef utputefficiency'at the higher frequencies; 5

In another form' of my invention] the air radio Cabinet. therebyrequiring minimum spa e;

inside theiarge opening of the air 'chamber and coupled I to the smallthroat," thereby 'efi'ecting a considerable saving inspacef'requirerxientl" In still another form of my invention; I show" aneconomicaldesign of the" air chamber strue-' ture which can be"fabricated bymo'lding.,.two

symmetrical halves and then cementing them to gether.

The novel "features that'I consider characterls tic of my inventionare'set forth" with partic u lanty in the appended claims. itself,-however; both as to its organization and method of operation; aswell'asadvantages there- 1 of,- will best be'understoodfrom thefollowing 1 description of several 'embodiments'thereof, when readinconnection with theacc'ompanying draw ings. in which:

Fig.1 is an end view of a portable radioincorporating one' form" of myinvention."

Fig; 2 is a *sec'tlomtaken along. the li ne 2-2 I Y hicl ij hevibrating. sea ater-w ems-he r queaau,

the resonant frequency of theleud speakergdi'aphragm and also acts asan" exponential "horn.

The invention, V

Fig. 6 is asection taken along the line 6 of Fig. 5.

Fig. 7 is a section taken along the line 'i7 of Fig. 6.

Fig. 8 is a graph showing the measured improvement in the performancecharacteristics of one type of loud speaker built in accordance with oneof the teachings of my invention.

Referring more particularly to the figures in which the same referencecharacter represents the same part throughout:

Figs. 1, 2, and 3 show three views of one embodiment of my invention;The reference character 1 represents the outer case of a portable typeradio receiver. Instead of employing a conventional arrangement for theloud speaker in which the sound is radiated directly from the diaphragmthrough a grill placed on one wall of the radio cabinet, I provide atapered air chamber which is designed to act as an exponential horn atthe higher frequencies, resulting in improved efiicien-cy for the loudspeaker 2 at these higher frequencies. The air column is also designedso that at the low frequencies, it acts as a sufficiently large massreactance load on the loud speaker diaphragm 3 to cause a substantialreduction in the resonance frequency of the Vibrating diaphragm. Inorder to achieve this dual function for the air'chamber, I found itnecessary to make the developed length of the air column less thanone-quarter wavelengthof the lowered frequency of resonance of thediaphragm and air column combined, and I also found it desirable to givethe air column an approximate exponential taper so-that it behaveseffectively as an infinite exponential horn at the higher frequencies.This results in the elimination of the mass reactance air load on thediaphragm at these higher frequencies and also results in an increase inthe acoustic efficiency of the loudspeaker at these higher frequenciesbecause of the increased acoustic radiation resistance which is imposedon the vibrating diaphragm by the exponential horn attachment.

In the form of my invention shown in Figs. 1, 2, and 3, the tapered airchamber is formed by several'adjacent walls of the radio cabinet and thefabricated portions 4, 5, 5, l, and 8. The strip 1 shown in section inFig. 2 and in partial section in Fig. 3, is joined to a pair of oppositeouter walls of the radio housing I to form the relatively largefunnel-like mouth opening of the air column which discharges the sound,in this particular instance, through the sound transparent grill 9 whichcovers one end of the radio cabinet, as indicated. The flat portion 6flares in a plane at right angles to the direction of fiare of theportion 7, as is indicated in the plan view of Fig. 3. The portion 6 ofthe horn structure, in combination with a pair of side walls, one ofwhich, 8, is shown in Fig. 2, forms a second portion of the tapered airchamber of the loud speaker system. The final portions 5 and 4 completethe tapered air chamber which is continuously expanding from its smallthroat opening shown adjacent to the diaphragm 3 to its large mouthopening shown adjacent to the sound transparent grill 9. An additionalflat portion l0 shown in section in Fig. 2 provides a flat platformaround the small throat opening of the air column to permit the loudspeaker 2 to be mounted upon it without air leakage between thediaphragm 3 and the throat opening of the air chamber. Angular blocks IIare placed in the corners of the right-angle bends, as indicated in Fig.2, in order to minimize the discontinuity in the rate of increase of thecross-sectional area of the air chamber at the particular bends. Thereference character 12 illustrates a carrying handle for the radio setwhich may be mounted, as illustrated. Although the design shown in Fig.2 employs some of the walls of the radio cabinet as part of the aircolumn structure, it would obviously be possible to fabricate the airchamber as a separate complete unit, if desired, and then mount it inthe same relative position, as indicated in Fig. 2. Such a construction,however, would add thickness to the radio cabinet and it would alsoincrease the cost of production. The space which remains within thecabinet I outside of the horn structure may be used for housing theradio set proper and batteries in any manner desired. The radio chassisand batteries are not shown in these figures since no claim is made toany particular arrangement of these parts.

For the ordinary loud speaker generally employed in small-size portableradio sets, the resonant frequency of the small diaphragm is generallyin the vicinity of or greater than 300 cycles. As a result of this highresonant frequency, which results inherently from the small diaphragmsize, the sound reproduction is lacking in low-irequency response, withthe result that the quality of reproduction is very thin. Also, becauseof the small radiating area of the diaphragm, which in many cases isless than that of a piston having a 2diameter, the efficiency ofreproduction is extremely low, which means that in addition to the lackof low-frequency tone reproduction, the volume of sound reproduction isalso weak, especially for those sets which are designed for economicaluse of batteries. By employing a tapered air column designed accordingto the teachings of this invention such that the developed length of theair column is less than one-quarter wavelength at the lowest frequencyof reproduction, I am able to effectively load the loud speakerdiaphragm 3 with a relatively large mass of air such that the resonantfrequency of the diaphragm is very substantially reduced. In oneembodiment of my invention, for example, in which the air chamber asshown in Fig.2 was adjusted so that its developed length wasapproximately 8", the area at the small end of the chamber was onehalfsq. in., and the area at the large end of the tapered column was 8 sq.in., I was able to reduce the resonant frequency of the diaphragm, whoseeffective vibrating diameter was in the neighborhood of 1% inches, byabout cycles, thereby adding a little more than one-half octaveadditional low-frequency range to the loud speaker reproduction. Bydesigning the air column to have an approximately exponentiallyincreasing cross-section, the air chamber becomes an efllcientexponential horn for the loud speaker at the higher frequencies andthereby serves to materially increase the efliciency of reproduction.These combined advantages of the speciallydesignedair chamber areclearly evident from the experimental response curves shown in Fig. 8.The dotted curve in Fig. 8 shows the meas- 193d "5 9 15 ha a te is o he;ouds ak r h n. un e gnv na yo po i e a r l opening on one wall of asmall radio cabinet. The solid curve in Fig. 8 showsthe measuredresponse characteristics of the same loud speaker arranged asillustrated in Fig. 2 and coupled to the specific-air chamber abovedescribed. The great improvement in low-frequency reproduction as wellas the indicated increase in the efflciency at the higher frequenciesactually produced a most spectacular improvement 'ir; the quality ofreproduction from the small portable radio. The reproduction was veryconsiderably louder and muchrnore pleasing in tone balance because .ofthe e t i d w-f equ ncy, range.

The important principle which achievesthe added low-frequency response.without sacrifice in high-frequency performance 7. is that. in my designof the air chamber, I. add mass to. the loud speaker diaphragm onlyatthe low frequencies, thereby reducing the diaphragm resonance. At thehigher frequencies, the air chamber becomes effectively a longexponential horn and the load on the diaphragm becomes essentially aresistance and the mass reactance disappears. In this way, the mass loadis eliminated at the higher frequencies and the acoustic efficiency ismaterially increased.

Figs. 5, 6, and 7 illustrate another embodiment of my invention. Fig. 5is a view similar to Fig. 1 except that the grill cloth 9 and thecarrying handle I2 have been removed. Fig. 6 illustrates the essentialdifference between this second embodiment of my invention over theprevious embodiment shown in Figs. 1, 2, and 3. The portion I 3 in Fig.6 runs from wall to wall in the radio cabinet I the same as was the casewith portion 1 in Fig. 2. The portion 6 in Fig. 6 is the same as theportion 6 shown in Figs. 2 and 3. The side pieces 8 shown in Figs. 5, 6,and 7 combine with the portion 6 to form a tapered horn section in thesame manner as part 6 and 8 were employed in the construction shown inFigs. 2 and 3. Parts 4 and I serve to complete the sealing of the smallend of the air column so that the loud speaker 2 may have its diaphragm3 coupled to the air chamber exactly in the same manner as was describedfor the corresponding parts in Figs. 2 and 3. The essential differencebetween the structure in Figs. 5, 6, and 7 as compared with thestructure in Figs. 1, 2, and 3 is that the tapered air chamber in Fig. 6is made re-entrant and, in this way, provides a different space factorfor accommodating the radio set components than results from theconstruction of Fig. 2. Although the construction illustrated in Figs.5, 6, and '7 utilizes some of the walls of the radio cabinet for part ofits structure, it is likewise possible to fabricate the re-entrant hornas a separat unit and then install it within the radio cabinet. Onemethod for economically fabricating the re-entrant horn is illustratedin the perspective view of Fig. 4. In this design, the bell-shapedportion l4 surrounds a second portion l5 which has side walls I6 and anopening ll such that when a symmetrical mating half, having the sameshape as Fig. 4, is joined to the cross-sectioned face, a completere-entrant horn similar to that illustrated in Figs. 5, 6, and 7 will beproduced. The type of design shown in Fig. 4 can be very economicallyproduced in molded plastic or molded paper pulp. The horn shown in Fig.2 could likewise be molded as a separate complete unit, if so desired,and then assembled within the carrying case I.

Although I havechosen certain speciflcnems bodiments of. my. inventionforv illustrating the basic features of myinvention, it. will beobvioustonthoseskilled in the art that numerous departuresmay be. made in. thespecific details forv executing the required. functions .and I,therefore, desire that my invention shall not be limited except-insofarasis madenecessary by the prior. art. and by. the.- spirit of the,appended claims.

I'claim asmy invention:

1. In combination in. a sound reproducing system, a loud speakerincluding .ayibratile diaphragm whose ,resonantfrequency lies in theapproximate range .200; cycles persecond to 400 cycles. per second,means for substantially reducing. the resonant frequencyof saiddiaphragm said means. comprising a tapered horn coupled to saiddiaphragm, the cross-sectional area of said tapered horn increasingalong the horn axis such that the area of the open end of said horn isgreater than the area of the horn in the vicinity of the diaphragm, thelength of said tapered horn being greater than the diameter of thediaphragm and less than A wavelength at the reduced resonant frequencyof the diaphragm.

2. The invention set forth in claim 1 further characterized in that theeffective vibrating area of the diaphragm is less than four squareinches.

3. The invention set forth in claim 1, further characterized in thatsaid horn has a cross-sectional area which increases along its lengthapproximately in accordance with an exponential law whereby said hornbehaves essentially as a long exponential horn at the higher frequenciesof sound reproduction, thereby serving to increase the acoustic emciencyof said vibratile diaphragm in said higher frequency region.

4. In combination in a small size radio set of the portable type, aloudspeaker having a fundamental resonant frequency within the approximaterange 200 cycles per second to 400 cycles per second, means forsubstantially reducing the resonant frequency of said diaphragm saidmeans Y comprising a tapered horn coupled to said diaphragm, thecross-sectional area of said tapered horn increasing along the horn axissuch that the area of the open end of said horn is greater than the areaof the horn in the vicinity of the diaphragm, the length of said taperedhorn being greater than the diameter of the diaphragm and less thanwavelength at the reduced resonant frequency of the diaphragm.

5. The invention set forth in claim 4, further characterized in thatsaid horn is constructed with some of the walls of the radio set servingalso as walls for said air column.

6. The invention set forth in claim 4 further characterized in that saidhorn has a cross-sectional area which increases along its lengthapproximately in accordance with an exponential law whereby said hornbehaves essentially as a long exponential horn at the higher frequenciesof sound reproduction thereby serving to increase the acousticeillciency of said loud speaker.

'7. The invention set forth in claim 4 further characterized in thatsaid horn has a cross-sectional area which increases along its lengthapproximately in accordance with an exponential law whereby said hornbehaves essentially as a long exponential horn at the higher frequenciesof sound reproduction thereby serving to increase the acousticefliciency of said loud speaker, and further characterized in that thelarge opening 7' of said horn terminates at one-endof the radio setthrough which end the sound is radiated.

8.. The invention set forth in claim 4 further characterized inthat-said horn has a cross-sectionalarea which increases along itslength approximately in accordance with an exponential law Wherebysaidhorn behaves essentially as a long exponential horn at the higherfrequencies of sound reproduction thereby serving to increase theacoustic efiiciency of said loud speaker, and further characterized inthat said horn is folded back on itself such that the'larger portion ofthe horn surrounds the smaller end portion.

9. The invention set forth in claim'4 further characterized in that saidhorn has a cross-sectional areaa which increases along its lengthapproximately in accordance with an exponential law whereby said hornbehaves essentially as a long exponential horn at thehigher frequenciesof sound reproduction thereby serving to increase REFERENCES CITED Thefollowing references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,730,425 Harrison i Oct. 8, 19291,878,018 Stephens Sept. 20, 1932 1,984,542 Olson Dec. 18, 19342,035,108 Weil Mar. 24, 1936 2,338,262 Salmon Jan. 4, 1944 Knowles May10, 1949

